It's 3.5 if you don't account for critical rolls, Natural 1's always missing and Natural 20's always hitting bring the value to +5.
It wouldn't be that bad if it was just advantage, but advagtageFOR/disadvantageAgainst is a net value of 10.
If it was just one or the other, I doubt forum members would be writing about high ground as often as we have.
I'm not sure even that is true. When you can make redundant whole spells or class abilities, in a game which depends on both, you are cutting out potentially vast amounts of balancing content for 'i'm higher than you are'.
